The invention relates to the level measuring instrument for measuring a level of a filled material in a container, in the case of which an electronic circuit generates electromagnetic signals which are transmitted via an injector to a waveguide projecting into the container. Said waveguide guides the signals into the container and guides out signals reflected at a filled material surface. The reflected signals are fed to a receiving and evaluating circuit which determines the level therefrom.
In this case, the waveguide can be provided both by a single waveguide and by two or more waveguides which are arranged parallel to one another and extend downwards into the container from a point above the highest level to be measured. Suitable as waveguides are, for example, bare metal wiresxe2x80x94also referred to as Sommerfeld waveguidesxe2x80x94or metal wires provided with an insulation. The latter are also known under the designation of Goubau waveguides.
Level measuring instruments operating with electromagnetic signals can be used in a multiplicity of applications, both in storage and in the processing industry, for example in chemistry, in the food industry and in the oil industry.
DE-U 94 21 870 specifies a level measuring instrument for measuring a level of a filled material in a container and which comprises:
an electronic circuit for generating electromagnetic signals,
a waveguide projecting into the container,
which guides the signals into the container and guides out signals reflected at a filled material surface, and
an injector,
which transmits the signals from the circuit to the waveguide, and
which has an inner conductor.
An electronic circuit for generating the electromagnetic signals, and receiving and evaluating circuits are described, for example, in EP-A 780 665.
In the case of the level measuring instrument in accordance with DE-U 94 21 870, the signals are short electromagnetic pulses which are reflected at the surface of the filled material. The injector is a coaxial bushing with an inner conductor and an outer conductor. A first side of the bushing is connected to the electronic circuit via a coaxial line. The inner conductor merges into the waveguide on an opposite, second side of the injector.
The characteristic impedance of the coaxial line is usually 50 xcexa9, that of the waveguide is a function of frequency and is, for example, a few 100 xcexa9.
At the transition from the pure coaxial conductor to the waveguide, a funnel coaxially embracing the waveguide is therefore integrally formed on the outer conductor in the case of the level measuring instrument in accordance with DE-U 94 21 870. Said funnel serves the purpose of avoiding impedance discontinuities and reflections in the region of the transition.
The funnel can serve the purpose not only of impedance matching, but also of improving the matching of the propagation modes.
The dimensions of a funnel are to be selected as a function of the wavelength of the electromagnetic signals. The larger the wavelength, the larger also must the funnel be constructed so that it achieves the desired matching of impedance and/or modes. However, short electromagnetic pulses regularly have not only one frequency but a frequency spectrum with a bandwidth. The spectrum is bounded below, for example by a direct component with 0 Hz. The upper bound depends on the type of pulse generation. In the case of a frequency spectrum from, for example, 0 Hz up to 1.5 GHz, the shortest occurring wavelengths are at approximately 0.2 m. There is no upper bound for wavelengths which occur. The funnel can be designed only for one of the frequencies occurring. Since a very large proportion of the energy in the case of a pulse is contained in signal components of low frequency, the funnel would have to be very large in order to offer adequate impedance matching at least for these components. If the funnel is large enough for the low frequencies, it is also suitable for higher frequencies.
Even a large funnel does not offer optimum matching, and it requires a great deal of space. However, this space is frequently not available, for example when there is only a small opening in the container for the level measuring instrument. These level measuring instruments are therefore regularly used without funnels.
It is an object of the invention to specify a level measuring instrument for measuring a level of a filled material in a container, in the case of which electromagnetic signals generated by means of an electronic circuit are guided into the container via a waveguide and signals reflected at a filled material surface are guided out, in the case of which an impedance-matching device active in a frequency band of a bandwidth is connected upstream of the waveguide.
For this purpose, the invention consists in a level measuring instrument for measuring a level of a filled material in a container, which comprises:
an electronic circuit for generating electromagnetic signals,
a waveguide projecting into the container,
which guides the signals into the container and guides out signals reflected at a filled material surface, and
an injector,
which transmits the signals from the circuit to the waveguide, and
which has an inner conductor,
which inner conductor has, for the purpose of increasing the characteristic impedance, a conductor section with a geometry deviating from a cylindrical shape.
In accordance with a development of the invention, the conductor section is a helical conductor.
In accordance with a refinement, the number of turns which the helical conductor has per unit length increases in the direction facing the waveguide.
In accordance with a refinement, the helical waveguide has a mean number of turns per unit length which is selected such that the characteristic impedance of the injector in the region of the conductor section is approximately equal to the square root of a product of the output characteristic impedance of the electronic circuit and the characteristic impedance of the waveguide.
In accordance with a refinement, the injector is a coaxial bushing with an inner conductor and an outer conductor, which has a first side connected to the electronic circuit via a coaxial line, and which has a second side at which the inner conductor merges into the waveguide.
In accordance with a refinement, the injector serves the purpose of mechanically fastening the level measuring instrument at a measuring place, and of fastening the waveguide.
In accordance with a refinement, the level measuring instrument has a receiving and evaluating circuit which receives the reflected signals and determines the level therefrom.